There exists in some single mode optical fibre systems a need for fixed attenuators. One method by which such an attenuator may be produced is described in published UK Specification No. 2128766A, and involves the localised heating of a single mode fibre to promote diffusion across the core/cladding interface in that region to produce a localised weakening of the strength of waveguiding, and hence an increase in attenuation. The amount of attenuation provided by this method depends upon the length of the heated zone, the temperature of heating, and the duration. This value can be increased by further heating, but no method is described by which it can be reduced.